The invention relates to a method and system for enhancing the spatial resolution of a fiber optical Distributed Acoustic Sensing (DAS) assembly.
International patent application WO2007/049004 discloses a Distributed Acoustic Sensing (DAS)assembly for sensing and monitoring traffic along several kilometers of the length of a road by means of an fiber optical cable buried alongside the road. In the known DAS assembly a series of light pulses are transmitted through the fiber optical cable by a light transmission and receiving assembly arranged at or near one end of the cable and back reflections of the transmitted light pulses are received by means of an interrogator assembly arranged at or near said end.
Utilising an optical fiber as an acoustic or vibration sensor can be achieved in a number of ways. One method is to launch a pulse of coherent laser light into a fiber. As the pulse travels through the fiber imperfections in the crystal lattice making up the fiber cause light to be reflected back along the fiber and dispersed out of the fiber. Under normal conditions, say for communications purposes, these back reflections are loss terms. However, the nature of the reflection causing imperfections are a function of the strain state of the fiber and as such by measuring the intensity of the back reflections and with multiple pulses it is possible to determine the strain state of the fiber as this varies temporally. Therefore an acoustic or vibration source which changed the strain state of the fiber could be measured using the back reflection data.
The launched laser pulse is precisely timed such that it's length in the fiber is known (10 m is a possible value for the pulse length). Once the pulse is launched the back reflections are measured. The measurement is made with a photodetector, which forms part of a light pulse transmission and receiving assembly and which integrates or adds up the number of photons received in a time period giving a figure relating to the total intensity of back reflected light. The time period can be matched to the laser pulse length and by using multiple contiguous readings will provide a measurement of how the back reflected light varies over the length of the optical fiber. Further by launching laser pulses in close succession and at a fixed rate (for example about 10000 pulses per second) a discretized representation of the change in strain state of the optical fiber as a function of both time and space can be achieved.
It is possible to reduce the length of the laser pulse to 5 m in the fiber. This also allows the spatial resolution to be improved to a 5 m channel spacing. However, the pulse length reduction causes a linear reduction in the energy (half the length=half the energy), which in turn reduces the level of back reflected light and leads to a worsening of the Signal to Noise Ratio(SNR) and therefore sensitivity of the system. There is a need to provide an improved Distributed Acoustic Sensing (DAS) method and assembly with enhanced spatial resolution, which does not reduce the level of back reflected light, the Signal to Noise Ratio (SNR) and/or sensitivity of the DAS method and assembly.